1. Field of the Invention
The present invention relates to a method of effectively eliminating phosphorus and/or antimony from molten aluminum made from a raw material containing phosphorus and/or antimony, such as non-reclaimed aluminum mass usually containing not less than 5 ppm of phosphorus or aluminum scraps, the method being applicable to a typical refining process.
2. Description of the Related Art
Recently, appeal for recycling of resources is becoming more and more intensive with increasing public opinion on environmental issues. In view of this, laws concerning recycling have been enforced in some countries including Japan. Accordingly, the industrial world is required to take measures for recycling without delay. The reclaimed aluminum industry has been positively promoting recycling of aluminum since the time before such a recycling movement arose. As a result, the proportion of aluminum scraps, such as city wastes and burr materials, included in the raw materials to be reclaimed is increasing.
Among various aluminum alloys, for example, hypoeutectic or eutectic Al-Si castings and aluminum alloys for diecasting having superior castability, strength and wear resistance such as AC3A, AC4A, AC4B, AC4C, AC8A and AC8B prescribed by Japanese Industrial Standard (JIS) can be modified by refining eutectic silicon therein with use of a modifier such as Na, Sb or Sr. Such modified alloys are used in great quantities as materials for component parts of vehicles such as brake drums, crank cases and pistons as well as of industrial machines, aircraft, household electric appliances and the like. Since these hypo-eutectic or eutectic Al-Si alloys have a broader allowable range of impurity elements, a great quantity of aluminum scrap is used to form a molten aluminum in the production of such an alloy. In the production of AC4CH, which is used in a great quantity for important safety-ensuring parts such as vehicle wheels, non-reclaimed aluminum mass is used in a large amount because AC4CH has a narrower allowable range of impurity elements.
Even a non-reclaimed aluminum mass having a purity of not less than 99.7%, which is often used industrially, contains phosphorus in an amount of about 5 to 15 ppm, and a Cu material and an Si material to be added in the production of an aluminum alloy also contain phosphorus. Accordingly, an aluminum alloy produced using such a non-reclaimed aluminum mass as a raw material contains phosphorus in an amount of about 5 to 20 ppm. Examples of aluminum scraps for use as raw materials of reclaimed aluminum include an aluminum scrap comprising an aluminum plate or sheet plated with Nixe2x80x94P, a hyper-eutectic Alxe2x80x94Si alloy containing phosphorus as added, an aluminum can, and vehicle parts of cast aluminum. Such aluminum scraps contain phosphorus and other impurities. Aluminum materials supplied as scraps generally contain phosphorus in an amount of about 5 to 100 ppm or more. Further, a Cu material and an Si material added in the production of an aluminum alloy also contain phosphorus. Thus, the content of phosphorus contained in resulting reclaimed aluminum is inevitably high.
When the content of phosphorus in an aluminum material is 5 to 10 ppm or more, refinement of eutectic Si is inhibited despite addition of a modifier, such as Na or Sr, and, hence, the efficacy of the modifier in enhancing the strength or the like is significantly reduced. An aluminum alloy made from such an aluminum material is unsuitable for casting or diecasting, will show an undesired etched state when subjected to a chemical treatment, will provide a product having a degraded surface quality, will cause a larger sink when cast, and suffers other problems caused by phosphorus.
As described above, phosphorus is an element affecting aluminum alloys for casting or diecasting. The mechanical properties, such as elongation and impact value, of such an aluminum alloy are improved when the content of phosphorus therein is not more than 5 ppm, more preferably not more than 3 ppm. Thus, reducing the content of phosphorus is critical in improving the quality of reclaimed aluminum.
Examples of presently known prior art approaches to overcome the foregoing problems include a method as described in Japanese Patent Laid-Open Gazette No. HEI 4-276031 wherein a molten aluminum at a specified temperature is filtered to remove Alxe2x80x94P compounds, and a method as described Japanese Patent Laid-Open Gazette No. HEI 7-2073066 wherein oxygen together with MgO is blown into a molten aluminum to produce a phosphorus oxide or a double oxide of Pxe2x80x94Mg, which in turn is separated off. Any one of these methods is not economic due to a large loss of aluminum and requires too much time to filter off such Alxe2x80x94P compounds, phosphorus oxide or double oxide of Pxe2x80x94Mg. For this reason, such methods are experimentally possible but have a poor feasibility as a fatal flaw because they are not applicable to any actual mass production.
Elements acting to deteriorate the mechanical properties of an aluminum alloy include antimony as well as phosphorus. Antimony is used as an additive for refinement of eutectic Si, and it is possible that aluminum scraps containing antimony are included in the casting materials. Antimony hinders the modifying effect of a modifier, such as Na or Sr, and hence is responsible for detective cast products having a sink or a reduced strength. Heretofore, a method of eliminating antimony from molten aluminum has not existed. Accordingly, all the aluminum alloys prepared from molten aluminum having inclusion of antimony have been judged as defective products, thus resulting in an increased cost. In addition, it has been impossible to completely separate aluminum scraps containing antimony from the casting materials.
Accordingly, it is an object of the present invention to provide a phosphorus and/or antimony eliminating method which can reduce a metal loss and does not require any filtering process, thereby ensuring a higher productivity.
According to the present invention, there is provided a method of eliminating phosphorus and/or antimony from molten aluminum containing phosphorus and/or antimony, comprising the step of adding magnesium or calcium to the molten aluminum maintained at temperature of 650xc2x0 to 850xc2x0 C. while blowing chlorine gas thereinto, to remove the phosphorus and/or the antimony contained in the molten aluminum.
According to the present invention, there is also provided a method of eliminating phosphorus and/or antimony from molten aluminum containing phosphorus and/or antimony, comprising the step of adding magnesium or calcium to the molten aluminum maintained at temperature of 650xc2x0 to 850xc2x0 C. while blowing a chloride thereinto, to remove the phosphorus and/or the antimony contained in the molten aluminum.
In any one of the above methods, magnesium or calcium is added to the molten aluminum for reaction with phosphorus and/or antimony contained therein to produce magnesium phosphide (Mg3P2) or calcium phosphide (Ca3P2), or Mg3Sb2 and a Caxe2x80x94Sb compound. Further, in the case of the former method, chlorine gas is blown into the molten aluminum for reaction with magnesium or calcium thus added to the molten aluminum to produce MgCl2 or CaCl2, which in turn absorbs magnesium phosphide or calcium phosphide, or Mg3Sb2 and the Caxe2x80x94Sb compound produced in the molten aluminum and surfaces to form dross, thereby reducing the contents of phosphorus and/or antimony in the molten aluminum.
Alternatively, in the case of the latter method in which a chloride, such as MgCl2 or CaCl2, is blown into the molten aluminum, the chloride thus blown surfaces while absorbing magnesium phosphide or calcium phosphide, or Mg3Sb2 and the Caxe2x80x94Sb compound.
MgCl2 or CaCl2 having absorbed magnesium phosphide or calcium phosphide, or Mg3Sb2 and the Caxe2x80x94Sb compound gathers on the surface of the molten aluminum to form dross, which in turn is removed from the molten aluminum. When the temperature of the molten aluminum is not lower than 850xc2x0 C., magnesium phosphide or calcium phosphide, or Mg3Sb2 and the Caxe2x80x94Sb compound becomes finer in the molten aluminum and, as a result, becomes hard to be absorbed by MgCl2 or CaCl2. Consequently, elimination of phosphorus and/or antimony from the molten aluminum becomes difficult. When the temperature of the molten aluminum is lower than 650xc2x0 C., MgCl2 or CaCl2 turns into a solid state from a molten salt state, with the result that elimination of phosphorus and/or antimony from the molten aluminum becomes difficult.
Examples of such chlorides include AlCl3, NaCl, KCl, CaCl2, BaCl2, LiCl, MgCl2, and C2Cl6. These may be used either alone or in combination of two or more of them. Though these chlorides are somewhat different in efficacy from each other, they exhibit similar phosphorus and/or antimony eliminating actions.